1. Field of the Invention
This invention relates to the field of recycling of polymers and in particular to densification, permanent compaction of expanded polymer materials, alternatively called foamed polymers, without binders or baling for economical transportation from collection points to remanufacturing sites for recycling.
2. Description of the Art
some have sought to recycle expanded (foamed) polymer materials in particles or large shapes by simply returning them to a manufacturer capable of recycling them. The difficulty with transporting expanded polymer materials is that they require enormous space in comparison to their weight. Their densities are so low as to waste the volume of a transport vehicle. That is, a truck, train or other transportation medium can "gross out" or "cube out" by receiving a load which meets the maximum weight or volume capacity, respectively, of the transportation medium. Expanded polymer materials "cube out" all transportation media but have minimal weight, rendering transportation uneconomical.
For example, trucks have a licensed weight limit and also a mechanical limit for the structure and the suspension. Ships have a gross weight limit dictated by the required freeboard above the water line. Trains have a maximum weight which the road bed and rails with intermediate supporting structure can support. On the other hand, any container, truck body, ship's hold or train car has a maximum number of cubic feet into which products can be placed. In hauling lead ingots, any transportation medium will be "grossed out" by the extremely dense ingots before it is "cubed out." Hauling styrofoam, a truck may "cube out" with less than one ton of weight in a forty-foot semi-trailer. Expanded polystyrene, styrofoam, typically has a density between 0.5 and 4.5 pounds per cubic foot, whereas steel has a density of almost 500 pounds per cubic foot. Moreover, the intrinsic value of steel per pound is high compared to the value of styrofoam and many recyclable polymers. A higher percentage of value is lost to transportation of recyclable polymers.
Most of the art of recycling polymer foams, alternately called expanded polymer materials, does not treat the transportation issue. Remanufacturing methods dominate the art. Patents deal with reforming or recasting ground waste materials. Polyurethane foam, shredded, remolded and re cut, is a good example.
U.S. Pat. No. 2,892,216 (Steel, 1959) discloses a method of recovering scrap from foam rubber by adding a polyurethane elastomeric binder to pieces of scrap polymeric material. Numerous binding and curing agents are mentioned with a feed screw for mixing. The maximum density disclosed is 13 lbs. per cubic foot from shredding of the scrap into crumbs, mixing it with the uncured binder, and molding the crumbs together until the binder cures by evaporation of a volatile solvent or by heat.
U.S. Pat. No. 3,452,122 (Stern et al., 1969) discloses a method of producing uniformly colored, foamed materials by means of using waste foam materials of various diverse colors bonded together with a polyurethane binder including a pigment and swelling agent which enables the pigment to penetrate substantially uniformly through even the waste foam which is also expanded.
U.S. Pat. No. 3,517,414 (Carson, 1967) discloses an apparatus for processing plastic material using scrap foam material cut into small pellets or pieces on the order of 1/8 to 1/4 inch maximum dimension. The pieces are mixed together with a plastic foam material which forms a binder. The mixture is cured in a mold to form a cylindrical body which is rotated about its axis while sheets are cut off in a thin continuous layer of recycled foam product.
Some recycling of expanded polystyrene, commonly called styrofoam, has been addressed. Remanufacturing is well known since polystyrene is a thermoplastic polymer rather than a thermoset one like epoxy or a reaction-cured polymer like polyurethane. However, polystyrene is severely hampered by its very virtues of light weight and high strength. It is not easily compressed and does not have enough density in most of its forms to make transportation to a central recycling center economical.
The prior art has relied on baling of light weight materials such as foamed polymers and cardboard boxes. Baling machines are known in the art and typically have a platen which compresses a stack or bin full of compressible materials against an anvil. After compression, bands or other retention means are placed around the slug of compressed material. The compressed material must then be transported as a bale to the location of re-use or other disposition.
Another method of recycling involves reusing polystyrene foam pellets in outgoing shipping for packaging material. That is, incoming packaging is emptied by large suction pipes which transport all the polystyrene pieces to a holding bin from which outgoing packages can be filled with beads or particles.
Some polymers, particularly elastomers such as polyurethane, are shredded and re-formed using binding elastomers spread or mixed among the shredded particles of recycled polymer foam.
U.S. Pat. No. 3,004,293 (Kreidl, 1959) teaches a method of compression of urea formaldehyde foam particles having a high moisture content and a bulk weight of less than two pounds per cubic foot without substantially impairing the cellular structure of the particles in the cohered mass. The method relies on adhesive binders as well as fillers.
U.S. Pat. No. 3,164,860 (Oxell, 1961) teaches an apparatus for uniformly mixing and charging a substance into a mold. The basic problem addressed is achieving uniform mixing of non-uniform particles when a lightweight particulate matter such as thermoplastic beads must be mixed with a heavier binding material. Particularly where the binding material is a liquid, lighter particles float above rather than mix in with the binder. Screw conveyers are used to promote mixing and to move the material from a hopper to a mold. Compression is only necessary to maintain good bonding between a binder and a granular material is used.
U.S. Pat. No. 1,803,814 (Spengler, 1928) discloses a process and apparatus for making pressed bodies of powderous material by uniaxial compression of a wetted powder for cohesion. The method teaches uniaxial compression heating, and open sides for escape of captured air. The method is a repeated layering and compression of a powdered material on a table. The process produces a batched block end product.
U.S. Pat. No. 2,307,371 (Hileman, 1941) discloses a method of introducing a frozen, moist, pulverized granular material into a mold and uniaxially compressing it to create a solid mass. The method relies on freezing granules, which melt at their interfaces due to the high pressure, and refreeze upon release of the pressure. An upper and lower plunger compress in the mold which is then emptied by one plunger, driving the formed article out.
Several patents specifically treat densification of expanded polystyrene (EPS), also known as styrofoam, for purposes of recycling. Although two principal processes of densification include compression and melting, extrusion may become principally melting in some of the apparatus disclosed. When extremely high pressures are applied for a short time, the process usually is one of adding work mechanically to the polymer material. The mechanical work, added at high pressure, is converted to thermal energy which melts the expanded polymer. The polymer exits and cools to a dense solid which must then be ground.
U.S. Pat. Nos. 4,436,682 (Knopp 1984) and 3,577,589 (Serrano 1969) disclose various apparatus which primarily extrude plastics. Knopp discloses a roller process which produces a sheet. Serrano discloses a radially moving feedstock of foamed thermoplastic extruded between rotating extrusion disks. The densified plastic is discharged as curds which are later ground.
U.S. Pat. No. Nos. 3,859,404 (Immel et al. 1975), 3,752,631 (Corbett et al. 1973) and 3,607,999 (Corbett et al. 1971) disclose various apparatus which melt a feedstock of plastic foam. Both the U.S. Pat. Nos. 3,752,631 and 3,607,999 to Corbett et. al. disclose radiant heating to melt foamed plastic. Immel discloses a method using steam to melt plastic foam.
Densifying expanded polystyrene foam with heat can be fast and continuous on a conveyor. Energy and space requirements can be substantial, with fumes an issue as well.
Heat quickly causes polymer chains to retract from their expanded, foamed positions. However, since the method is typically in open air as a practical matter, oxidation degradation of the polymers occurs. Degradation of the polymer treated is undesirable as it changes the chemistry of the polymer, changing the resultant material properties. Degraded polymeric chains become impurities virtually impossible to remove. The ability to be recycled is impaired.
Another method of compressing expanded polymers such as polystyrene includes compression within a container by a platen or wall of the container. Such a device has been produced with a hopper having a movable wall for compression.
The hopper is filled with polymer foam pieces and the platen, the one movable wall, applies a force to crush the material. The platen is repeatedly retracted, additional material is added and the platen crushes the net contained material. After several cycles, the platen is held in place for a time sufficient for the compressed polymer foam to take a permanent set and remain densified.
One of the major difficulties of the foregoing process is that the operation is too slow. The entire apparatus is occupied for each load as it is held for the required relaxation time, the time taken by the polymer foam to mechanically creep, take on a permanent set. The polymer comes out as a large block. Meanwhile the compression process is not very effective when the path of escape for air captured within the polymer foam is half the width of the face of the large platen required. The pressure on such a very large platen creates severe structural difficulties in a unit of commercially desirable size. Also, the large block is difficult to handle. A fork truck or similar equipment may be necessary.
Effective recycling technology for foamed polymers, should address expanded polystyrene which is used in numerous industrial and consumer applications. Polymer recycling needs an economical method of densification, permanent compression, which can obtain a density of 25-30 pounds per cubic foot without the use of binders, packaging or baling for restraint. Large expensive machinery for compression should be minimized and effectively used when required. The method must not cause thermal degradation of the polymer by excessive temperature excursions. The recyclable polymer, after compression, should be adaptable to efficient stacking by hand or machine. Transportation should be economical by truck to a recycling center. Recycling should include operators who can extrude pellets for resale to manufacturers as feedstock for molding machines.
Since the volume of expanded polymers drives up the cost of transportation, and cost is a great deterrent to recycling, compression as close to the site of collection as possible is preferred. Preferably the cost of a densification machine should be modest for proliferation near as many collection sites as possible.
Sites such as business receiving docks, fast food restaurants, and similar locations having a high-volume throughput of expanded polymer packaging are ideal sites for a densification system.
Capital cost will, of course, influence how widespread the installed machines become. Floor area, commonly called "footprint," is an extremely important element of cost. A commercially reasonable densification system should have minimum size. Similarly, minimum costs of operation, maximum safety for personnel, maximum reliability, a minimum number of parts, minimal precision requirements and low wear rates are desirable attributes of such a densification system.